1. Field of the Invention
This invention relates to fuel cell systems with at least one fuel cell stack and an external manifold and, in particular, to a gasket for use in a fuel cell system having at least one externally manifolded fuel cell stack. More particularly, the invention comprises an gasket that seals the manifold to the stack and accommodates differential movement between the stack and the manifold by allowing slippage at interfaces between them.
2. Description of the Related Art
A fuel cell is a device that directly converts chemical energy in the form of a fuel into electrical energy by way of an electrochemical reaction. In general, like a battery, a fuel cell includes a negative electrode or anode and a positive electrode or cathode separated by an electrolyte that serves to conduct electrically charged ions between them. In contrast to a battery, however, a fuel cell will continue to produce electric power as long as fuel and oxidant are supplied to the anode and cathode, respectively.
In order to produce a useful amount of power, individual fuel cells are typically arranged in stacked relationship in series with an electrically conductive separator plate between each cell. A fuel cell stack may be categorized as an internally manifolded stack or an externally manifolded stack. In an internally manifolded stack, gas passages for delivering fuel and oxidant are built into the fuel cell plates themselves. In an externally manifolded stack, the fuel cell plates are left open on their ends and gas is delivered by way of manifolds or pans sealed to the respective faces of the fuel cell stack. The manifolds provide sealed passages for delivering fuel and oxidant gases to the fuel cells and preventing those gases from leaking either to the environment or to the other manifolds. In some fuel cell stack arrangements, the stack is placed in an enclosure and the enclosure environment represents one of the process gases. In such a system, at least three manifolds are required to provide inlet and outlet gas passages for the stack, each of which must be sealed to the stack. In any case, the manifolds must perform the above functions under the conditions required for operation of the fuel cell stack and for the duration of its life.
An important aspect of the performance of an externally manifolded fuel cell stack is the seal established between the manifold edge and the stack face. The manifolds, which are constructed from metal, must be electrically isolated from the stack face, which is typically electrically conductive and has an electrical potential gradient along its length. Dielectric insulators are used between the metallic manifold and the fuel cell stack to electrically isolate the manifold from the stack and to prevent the manifolds from shorting the stack.
A fuel cell stack will usually shrink over its life as fuel cell components creep and densify at high temperature and pressure. Such shrinkage and changes in fuel cell stack dimensions create stresses on the manifold assembly during stack operation. In particular, vertical stack dimensions may change by as many as 2-3 inches in a stack of 300 or more fuel cells. As the stack changes in vertical dimension, it requires the seal between the dielectric insulators in the manifold assembly and the stack face to accommodate movement along the end plates at the top and bottom of the stack. Differences in coefficients of friction between the dielectric and the seal and between the seal and stack face apply additional stress to the seal and may cause it to wear during stack operation and eventually fail. Therefore, there is a need for a manifold-stack seal that is better able to accommodate such dimensional changes while maintaining a gas seal and electrical isolation of the manifold from the stack.
The dielectric insulators, which are typically made from brittle ceramic materials such as alumina and mica, may be easily damaged by thermal and mechanical stresses applied on the manifold system during fuel cell stack operation. In order to withstand the stresses imparted on the manifold system while maintaining electrical isolation between the manifold and the stack, improvements have been made to the manifolds and to the dielectric insulators used to isolate them from the stack.
For example, a common dielectric insulator assembly is designed as a rectangular frame with joints that allow for differential movement between the stack and manifold. Such a construction is shown and described in U.S. Pat. No. 4,414,294 which discloses a rectangular insulator frame having a plurality of segments interconnected by slidable spline joints that permit expansion or contraction with the walls of the manifold and the fuel cell stack. In order to withstand stresses caused by differential movement of the fuel cell stack, the dielectric frames may be made from high-density ceramics. However, manifold compression against the stack face and stack compaction during operation of the fuel cell stack cause mechanical stresses which are not completely accommodated by the ceramic dielectrics and may still damage them. There is thus a need for a manifold-stack seal that is better able to accommodate such thermal and mechanical stresses and prevent the ceramic dielectrics from breaking.
In some contemporary manifold systems, the manifold assembly is compressed against the stack so that the seal between the manifold and stack is maintained when changes in the stack dimensions occur. One such stack manifold compression assembly is shown and described in U.S. Pat. No. 4,467,018 which discloses external reactant manifolds strapped or clamped to the fuel cell stack. U.S. Pat. No. 6,461,756 describes a retention system for maintaining external manifolds in sealing relationship to the fuel cell stack. A flexible manifold system that conforms to the stack shape and accommodates stack movement during operation is described in co-pending U.S. patent application Ser. No. 10/264,866, filed Oct. 4, 2002, assigned to the same assignee hereof.
In addition, various other components of the fuel cell stack and manifold sealing assembly have been further improved to address the limitations of the seal between the manifold and stack. For example, improved designs of bipolar separator plates in fuel cells, which provide flat sealing surfaces for the dielectric frame and gasket, are described in several patents, including U.S. Pat. No. 4,514,475 which teaches a fuel cell separator plate that can adjust to changes in thickness of cell parts during use; U.S. Pat. No. 5,399,438 which teaches a stainless steel member with high corrosion resistance; and U.S. Pat. No. 5,773,161 which teaches an improved bipolar separator structure that assists in electrolyte management by providing trough areas for dispersal or absorption of electrolyte. A gasket design to accommodate the growth of the bipolar plates over time during operation of the fuel cell stack is described in co-pending U.S. patent application Ser. No. 10/627,035, filed Jul. 25, 2003 and also assigned to the same assignee hereof. Particularly, a compressible gasket and dielectric gasket provide a seal between the dielectric and the manifold. The compressible gasket additionally provides a compliant member embedded therein that conforms the gasket to the dielectric joints and manifold irregularities. The ceramic dielectric frame described in co-pending U.S. patent application Ser. No. 11/020,592 filed Dec. 23, 2004 and assigned to the same assignee hereof, enhances dielectric isolation and comprises an interlocking arrangement that reduces mechanical stresses and prevents wear on the abutting gaskets. However, these improvements in the manifold assembly, fuel cell stack components, and dielectric insulator construction do not address the differences in coefficients of friction at the interfaces between the dielectrics and the seal and between the seal and stack face and the resulting wear on the manifold gasket.
Another consideration is that fuel cells operate at very high temperatures. For example, molten carbonate fuel cells operate at about 650° Celsius. The selection of materials to be used in the manifold gasket must account for this long term operating temperature and allow the components to last for the life of the fuel cell stack, which is typically several years. There is therefore a need for a manifold seal that tolerates fuel cell stack operating temperatures and can accommodate stack movement and changes in stack dimensions.
Another limitation of the seal between the dielectric insulator and the stack face is that it may permit undesirable movement of electrolyte from the top or positive end of the fuel cell stack to the bottom or negative end of the stack, which would flood the cells at the negative end of the stack and deplete electrolyte in cells at the positive end. The gasket may also permit electrolyte migration from the stack across the dielectric to the manifold. Methods and devices for reducing or mitigating electrolyte migration in fuel cell systems have been discussed in U.S. Pat. No. 4,643,954 which discloses a passageway along the height of a fuel cell stack with electrolyte wettable wicking material at opposite ends thereof, which equalizes molten electrolyte content throughout the stack; U.S. Pat. No. 4,761,348 which teaches an electrolytic fuel cell stack having a combination of inactive electrolyte reservoirs at the upper and lower end portions that mitigate the ill effects of electrolyte migration, and a porous sealing member with low electrolyte retention that limits electrolyte migration; and U.S. Pat. No. 5,110,692 which teaches a manifold gasket for molten carbonate fuel cells having an elongated porous member that supports electrolyte flow and barrier means for retarding such flow, for controlling electrolyte flow and reducing electrolyte migration. Highly polished ceramics, such as those described in U.S. Pat. No. 6,514,636, are also desirable for providing the required voltage isolation by preventing or reducing electrolyte creep over the surface of the frame.
It is therefore an object of the invention to provide a fuel cell manifold sealing assembly including a gasket for sealing a manifold to the face of a molten carbonate fuel cell stack that accommodates differential movements resulting from thermal stresses and internal fuel cell compactions during operation of the fuel cell stack, while maintaining a gas seal between the manifold and stack and keeping the manifold electrically isolated from the stack.
It is a further object of the invention to provide a gasket that accommodates differential movement at the interfaces between a manifold and fuel cell stack and reduces stresses on the dielectric insulator component of the gasket assembly.
It is yet another object of the invention to provide a gasket that accommodates differential movement at the interfaces between the manifold and fuel cell stack while limiting electrolyte migration.